In digital systems, it is often necessary to determine the relationship between two frequencies in order to provide control information for system frequency adjustments. This relationship is determined by transmitting two signals, a reference signal of a known frequency and a variable signal of an unknown frequency, to a frequency and phase comparator. The frequency and phase comparator monitors these signals and provides frequency difference information.
One type of frequency difference information is the detection of slips between the two signals. A slip indicates that during transmission the two signals are not always of equal frequency, but at times the variable signal changes frequency with respect to the reference signal. Assume that the two signals are squarewaves having transitional points corresponding to the leading and trailing edges of the squarewave pattern associated with each signal, and that these edges are consistently identifiable for each signal throughout transmission. Assume further that the leading edge is the preselected transition point for both signals which the frequency and phase comparator monitors. Therefore, when the leading edges of both signals coincide, the two signals are in phase and of equal frequencies. It is the function of the frequency and phase comparator to detect this coincidence between the leading edges of each signal.
The frequency and phase comparator detects slips when there is a lack of coincidence between the leading edges of each signal. In particular, when the variable signal changes frequency with respect to the reference signal such that the two signals are close in frequency but not equal, the leading edges of the variable signal drift with respect to the leading edges of the reference signal. The two signals are now also out of phase with respect to one another. The two signals remain out of phase until the leading edges of the variable signal drift back to the point where all the leading edges of both signals coincide. This indicates that both signals are of equal frequency and in phase again.
The above-described situation where the two signals are out of phase with respect to each other for some duration of time is identified as a slip. A full slip occurs when the leading edges of the variable signal drift 360 degrees before all the leading edges of both signals again coincide. A full slip identifies a time during transmission when the frequency of the variable signal did not equal the frequency of the reference signal. The frequency and phase comparator records the number of full slips and produces a slip rate indication. The slip rate represents the number of times the variable signal changes frequency with respect to the reference signal during transmission.
Another type of adjustment information is a difference frequency indication. The difference frequency specifies the direction of drift of the variable signal as well as the magnitude of the frequency change. The direction of drift identifies whether the frequency of the variable signal is higher or lower than the frequency of the reference signal over the duration of each slip.
In determining a slip rate or a difference frequency indication, it is not necessary to use only squarewaves. Both signals may be represented as any arbitrary waveform, such as triangular, sawtooth or rectangular. The only requirement is that the waveforms have a consistent identifiable transition point which can be monitored by a frequency and phase comparator during transmission.
There are several problems associated with the currently available comparators. For example, the comparator disclosed in U.S. Pat. No. 3,663,884 issued to Pattantyus on May 16, 1972 produces a difference frequency indication only when the variable signal is higher in frequency than the reference signal. The obvious disadvantage of this comparator is that there is no difference frequency output when the frequency of the variable signal is lower than the frequency of the reference signal.
Another example is found in U.S. Pat. No. 4,264,866 issued to Benes on Apr. 28, 1981. The Benes frequency and phase comparator produces a difference frequency indication that is distorted by any noise or "jitter" which occurs in the variable signal as a result of unsustained abrupt changes in the monitored transition point of the phase. When the monitored transition point of the variable signal drifts more than 90 degrees with respect to the monitored transition point of the reference signal, the Benes comparator misinterprets the relationship of the transitional points between the two signals and produces an output that indicates a full slip even though a true full slip did not occur.
One further example is found in an article entitled, "A Cycle-Slip Detector for Phase-Locked Demodulators" by Floyd M. Gardner, which appeared in the IEEE Transactions on Instrumentation and Measurement, Vol. IM-26, No. 3, September 1977. The Gardner comparator fails to detect slips until the monitored transition point of the variable signal drifts 270 degrees with respect to the monitored transition point of the reference signal. Therefore, a first full 360 degree slip is not detected until an initial drift of 270 degrees. Slip detection is inefficient since time is wasted before a first slip is detected and a frequency change in the variable signal may result during this initial drift.